1. Field of the Invention
The present invention relates to a digital wireless communication system, and more particularly, to a digital cordless phone system which changes encoding and decoding methods according to the distance of speech communication between a remote unit and a base unit, and a method thereof.
2. Description of the Related Art
A digital wireless communication system provides better communication quality than that provided by a conventional analog wireless communication system. Particularly, a digital cordless phone system, which replaces the existing analog cordless phone system, uses encoding methods such as adaptive differential pulse code modulation (ADPCM) to provide a better voice quality via a low transmission rate or low transmission band, and uses a modem in transmitting encoded voice data. This digital cordless phone system provides diverse advantages. For example, the digital system provides tapping prevention function using encryption and an excellent speech quality within a preset speech communication distance. If the distance of current speech communication between a base set and a handset of the digital cordless phone system is within a preset speech communication distance, that is, within a speech communication distance having low bit-error rate, good speech quality can be maintained. Here, the base set and the handset are, for example, a base unit and a remote unit, respectively, of a digital cordless phone.
However, if the current speech communication distance is beyond the preset speech communication distance, bit errors increase due to degraded signal-to-noise ratio (SNR), and the speech communication quality provided by the digital cordless phone system is reduced.
Therefore, a need exists for a digital cordless phone system which provides good speech communication quality even when the current speech communication distance is beyond the preset speech communication distance.
To solve the above and other problems, it is an object of the present invention to provide a digital cordless phone system which can improve (or lengthen) the distance of speech communication between a base set and a handset, by using a first encoding method if the speech communication distance between the base set and the handset is within a preset speech communication distance, and using a second encoding method and error concealment if the speech communication distance between the base set and the handset is beyond the preset speech communication distance.
It is another object of the present invention to provide a voice communication method applicable to the digital cordless phone system to maintain a good quality in the speech communication regardless of the communication distance between the base set and handset.
To accomplish the above and other objects of the present invention, there is provided a digital cordless phone system having a base unit and a remote unit, in which each of the base unit and the remote unit having a first codec for encoding voice data in a first encoding mode and decoding data encoded in the first encoding mode in a first decoding mode; a data processing unit having a second codec for encoding the voice data in a second encoding mode, wherein the data processing unit adds an error-detection code to encoded data to indicate existence of an error in the encoded data, removes the error-detection code from encoded data received from one of the base unit and the remote unit, performs error concealment with respect to the encoded data from which the error-detection code is removed, and provides error concealed data to the second codec to decode the error concealed data in a second decoding mode; a controller for generating a selection signal for changing an encoding mode of the digital cordless phone system in response to a signal representing strength of a signal received from the base unit or the remote unit; a modem for modulating data output from the first codec or the data processing unit, and demodulating data remotely received from the base unit or the remote unit; and a switch for connecting the first codec or the data processing unit to the modem in response to the selection signal.
To accomplish another object of the present invention, there is also provided a voice communication method between a base unit and a remote unit in a digital cordless phone system, the method having the steps of (a) determining whether a current speech communication distance between the base unit and the remote unit is beyond a preset distance of speech communication; (b) if it is determined that the current speech communication distance is within the preset distance of speech communication, encoding voice data in a first encoding mode and decoding receiving data in a first decoding mode corresponding to the first encoding mode to restore the voice data; and (c) if it is determined in the step (a) that the current speech communication distance is beyond the preset distance of speech communication, encoding the voice data in a second encoding mode, adding an error-detection code to encoded data, removing the error-detection code from receiving data, and decoding the receiving data from which the error-detection code is removed in a second decoding mode corresponding to the second encoding mode to restore the voice data.